i really wouldn't panic sometimes they slow down eating for any number of reasons. It may be tired of the foods you feed it.
What is the temp you keep it at? somtimes it gets to cool or to hot it may stress the animal(this is not good). You can check with local vets, to see if they'll look at turtles, via phonebook but with turtles it is hard to tell. More then likely it is wild caught so it could be parsites look for any worms or things floating in the water when it passes a stool.

w